1. Technical Field
The present invention relates to a mobile communication system, a mobile terminal, a radio base station, a radio network controller, a method for controlling an information rate for the information transmitted to and from the mobile, and a program therefor. More particularly, the present invention relates to a mobile communication system in which the mobile terminal and the base station are in a mobile objects and the mobile terminal originates or terminates a call via the radio base station.
2. Description of the Related Art
Mobile communication systems are desired to allow user terminals to perform communication at any time in any place. The user terminals are mobile terminals and each of the user terminals must be assigned a communication channel irrespective of the moving states of the user terminals or the distribution thereof so as to perform communication at any time in any place.
In conventional mobile communication systems, user terminals can perform communication irrespective of whether the user terminals stand still, are carried by walking users, or stay on vehicles.
As an example of the system described above, the Japanese Laid-Open Patent Application No. 2001-44930 shows a communication system in which user terminals communicate with a radio base station that is installed on the ground or disposed in a building on a stationary basis. Further, as another example of the system above, the Japanese Laid-Open Patent Application No. 07-154856 shows a communication system in which a mobile radio base station such as a communications satellite communicates with user terminals while moving along an orbit or the like.
In these communication system shown in the Japanese Laid-Open Patent Applications, even if a user terminal lies in a vehicle, a train or any other mobile object, the user terminal communicates with a radio base station located outside the mobile object. Accordingly, even though the user terminal does not move relatively to the mobile object, the user terminal is affected by the movement of the mobile object relative to the radio base station. Therefore, the user terminal lying in the mobile object and the radio base station disposed outside the mobile object on a stationary basis must have many control functions included therein.
Furthermore, in this case, the radio base station disposed outside the mobile object needs the same number of radio control units as the number of user terminals that lie in the mobile object. The radio control units maintain the communication with the respective user terminals that lie in the mobile object.
In order to improve a mobile communication system in terms of the problem described above, a communication system called Moving Network is proposed. In the communication network, the communication between a radio base station disposed outside the mobile object and the mobile object may be established via a base station in the mobile object using an integrated single radio link irrespective of the number of user terminals lying in the same mobile object.
The mobile communication system called Moving Network has been proposed in order to provide a mobile communication service, which is equivalent to that for user terminals outside a mobile object, for user terminals that lie in the mobile object, such as a train or a bus that moves on the ground, a marine vessel that moves on the sea and an airplane that moves in the air.
In the Moving Network, a base station is implemented in a mobile object that communicates with a base station located outside the mobile object. User terminals existing in a mobile object communicate with a base station in the mobile object rather than a base station outside the mobile object.
FIG. 22 schematically shows the moving network. Referring to FIG. 22, the moving network comprises a radio network controller 14, a radio base station outside a mobile object (B) 12-1, a relay base station 13, radio base station in the mobile object (B) 12-2, a user terminal outside the mobile object (B) 1-3 and user terminals in the mobile object (B) 11-1 and 11-2. The radio network controller 14 is connected to a plurality of radio base stations including the base station 12-1 and maintains communications with the user terminals while the user terminals move within a service area. The radio base station 12-1 is normally installed on a stationary basis. The radio base station 12-1 can be connected to the user terminals over a radio link.
The relay base station 13 within the mobile object B has a relay facility via which communication is established with the user terminals 11-1 and 11-2 lying in the mobile object B. The relay base station 13 includes a communication facility via which communication is established with the stationary radio base station 12-1, and a communication facility via which communication is established with the radio base station 12-2. The radio base station 12-2 establishes communication with the user terminals 1-1 and 1-2 in the mobile object B.
One of advantages of the moving network architecture is that a use efficiency of a radio link between the radio station in a moving object and a base station outside the moving object can be improved by employing a high-performance processing unit and high-precision antenna facilities in these base stations.
Another advantage of the moving network is that the user terminal needs to transmit radio waves with an intensity sufficient only to be propagated from a user terminal to the base station in the mobile object, providing an improved use efficiency of a resource of radio waves. This advantage is highly appreciated because, in most cases, the user terminal is equipped with an omni-directional antenna which has a directivity in all directions which transmits radio waves to all directions and may provide a deteriorated use efficiency of a radio resource of radio waves. Here, in most cases, the user terminal is equipped with an omni-directional antenna because the physical size of a user terminal is limited, the throughput of a processor employed is limited, and it is hard to determine the direction of radio waves propagated from the user terminal or the direction of radio waves to be received by the user terminal while the user terminal moves within a mobile object.
Next, a control method regarding the moving network for controlling transmit power of radio-waves and a information rate for communication between a user terminal and a base station in a moving object will be explained. A base station and a relay base station in a mobile object communicates with a radio base station disposed outside the mobile object within a service area covered by the mobile communication system. A communication link employed for general user terminals is used even for the communication between the relay base station and the base station outside the mobile object unless there is a communication link dedicated to the mobile object.
While a mobile object is moving, if the mobile object enters an area in which many user terminals perform communication, a radio link capacity usable by the mobile object diminishes. In addition, the link capacity may be changed due to a change in radio-wave propagation condition.
Where the radio link capacity diminishes, if transmission power for the user terminals in the mobile object to perform communication is held intact, interference power that adversely affects other user terminals in the mobile object increases. Consequently, an information error occurs at the user terminals. Therefore, power must be reduced by controlling transmission power according to the radio link capacity.
However, when transmission power is merely decreased, an information error occurs in information being transmitted to or received from the user terminals in the mobile object. In order to avoid information errors, an information rate must be lowered along with a decrease in the radio link capacity.
A information rate may be adjusted by changing a compression ratio as long as, for example, a voice service is provided using a compression voice coder/decoder. The information rate is usually varied depending on whether a speaker utters voice. However, the information rate may be varied when the capacity of a radio link is increased or decreased. For reference, when a compression ratio is increased, voice becomes coarse. On the other hand, when the compression ratio is decreased, voice becomes fine. If a shortage in a radio link capacity occurs, a speaker may sacrifice quality of voice and increase the compression ratio. Thus, the radio link capacity can be saved. However, a listening side cannot listen to clear voice. The foregoing treatment should preferably be performed because even when audio information exhibits a high information rate, if the audio information contains numerous information errors, normal voice cannot be reproduced. Moreover, although a radio link capacity diminishes, if communication proceeds at a high information rate, radio communication powers to be supplied to other user terminals will be interfered.
In the moving network, the relay unit interposed between the user terminals and the radio base station installed outside the mobile object is generally a mere relay facility. Therefore, normally, transmission power and a information rate may be controlled according to a control procedure described below.
That is, the control procedure comprises:
(1) Assessing a radio link capacity between a relay unit in a mobile object and a radio base station disposed outside the mobile object, and detecting a decrease in the capacity;
(2) Reducing transmission power so as to adjust the transmission power in line with the link capacity;
(3) Neglecting a reception error occurring in communication between the relay unit in the mobile object and the radio base station disposed outside the mobile object because the transmission power cannot be increased;
(4) Detecting occurrence of a reception error in communication between the radio base station and any of the user terminals in the mobile object (the reception error in information may occur at the user terminals independently of one another);
(5) Decreasing an information rate for the communication between each user terminal and the radio base station in the moving object through negotiation between each user terminal and a voice coder included in core equipment;
(6) Recognizing a decrease in the number of errors in communication between each user terminal and the base station in the mobile object;
(7) Observing a decrease in the number of reception errors in communication between the relay unit in the mobile object and the radio base station disposed outside the mobile object; and
(8) Continuing the communication at the information rate suitable for the link capacity between the relay unit in the mobile object and the radio base station disposed outside the mobile object.
According to the foregoing conventional control procedure, a capacity of a radio link between the mobile object and the radio base station disposed outside the mobile object is assessed in order to detect a decrease in the capacity. Thereafter, transmission power for communication between them is reduced, and then a user terminal experiences errors in received information due to the transmission power reduction. Finally, the user terminal reduces the information rate so as to reduce the number of the errors. Thus, the problem regarding the conventional control procedure is that it takes a long time to cope with a decrease in a link capacity and a user experiences defective communication until the information rate is reduced and the procedure is completed.